The invention relates to a device for burning in light-sensitive layers in the production of offset printing forms by heating the printing form by means of a radiation device which has an infrared fraction of about 50 to 87%, a fraction of about 10 to 30% in the visible region and an ultraviolet fraction of about 3 to 20% in the electromagnetic radiation.
In the case of positive-working light-sensitive layers, those parts of the light-sensitive layer struck by light during the imagewise exposure of the printing plate become relatively more soluble than the parts which have not been struck by light. As a result of the first mentioned parts being dissolved out in the developing step, the non-image areas of the later printing form, which are water-bearing on printing, are formed on the printing plate surface after the development of the exposed layer, and the image areas which are ink-bearing on printing are formed from the last-mentioned parts. In practice, it has been found that heating of the exposed and developed printing plate--to temperatures of about 180.degree. C. and higher, depending on the type of the support material--from the side of the light-sensitive layer or also from the back can lead to longer print runs; this operation is termed `burning-in` or `baking`. Two factors appear to be decisive in this regard. On the one hand, a mechanical consolidation of the image areas takes place, so that these are less susceptible to damage during storage and printing, for example by the action of light. On the other hand, the image areas also become more resistant to chemical or physical-chemical attacks, which manifests itself, for example, in that the image areas are less readily attacked by printing inks containing organic solvents or by etching media than image areas which have not been burnt in.
The following processes and/or devices for burning in light-sensitive layers have, for example, been disclosed in the state of the art:
German Offenlegungsschrift 31 10 632 (EP-B-0,061,059) discloses a process for burning in exposed and developed, positive-working light-sensitive layers, containing diazo compounds, in the production of offset printing plates by heating the printing plate. The electromagnetic radiation of the radiation source contains an infrared fraction of 50 to 87%, a fraction of 10 to 30% in the visible range and an ultraviolet fraction of 3 to 20%, the printing plate being heated at least to 180.degree. C.
In German Patent 854,890 (=U.S. Pat. No. 3,046,121), the aftertreatment of positive-working, light-sensitive layers containing diazo compounds by heating the printing plates carrying these layers is described. The heating is effected before or after the development of the exposed printing plate by treatment with flames, with a hot pressing iron, in a box electrically heated to elevated temperatures or between heated rollers; it is intended that the unexposed diazo compound is decomposed by the heating and the decomposition products are firmly bound to the support material.
German Auslegeschrift 1,447,963 (=British Patent 1,154,749) discloses a process for producing an offset printing form, wherein the printing plates having positive-working light-sensitive layers, which contain o-naphthoquinone-diazide compounds, are heated after development in the presence of a novolak and/or a resol to a temperature of at least 180.degree. C. The resin is either already present in the light-sensitive layer, or it is applied in solution to the light-sensitive layer before the heating. The level of the heating temperature and the heating time are selected such that, as the result of the decomposition of the ink-bearing image areas, not struck by the light, a precipitate is formed on those parts of the printing plate which have been struck by light, i.e. removed by the developer, and are water-bearing during printing. By means of an aftertreatment, these parts of the printing plate, which are water-bearing during printing, are then cleaned again. The heating temperature is especially 200.degree. to 250.degree. C. The corresponding heating time is 5 to 60 minutes, the heating being carried out in a burning-in stove.
The device for the burning-in of printing plates with an exposed and developed diazo layer according to German Patent 1,955,378 (=British patent 1,330,139) runs at a heating temperature of at least 180.degree. C. and contains, as the heating element, a quartz/halogen lamp having a radiation fraction of 10 to 15% in the visible spectral region. These quartz/halogen lamps are said to be distinguished by a relatively short heating-up time as compared with heating cabinets or infrared (IR) heating rods. As compared with IR heating rods, they have a considerably higher radiation fraction in the visible spectral region.
In the process for producing planographic printing forms according to German Offenlegungsschrift 2,201,936 (=British Patent 1,413,374), the exposed and developed light-sensitive layers are heated from the back of the printing plate by IR radiation, this back carrying an IR-absorbing layer.
In German Utility Model 6,901,603 (=U.S. Pat. No. 3,589,261) a device for developing negative-working, light-sensitive layers containing photopolymerizable compounds on flexible support materials is described, which also comprises (page 3, paragraph 4; page 5, paragraph 1, page 9, paragraph 4 and page 10) an aftertreatment stage, in which lamps emitting UV (ultraviolet) and IR light are used. In this stage, it is intended that, on the one hand, the material is dried after the main development stage and, on the other hand, an afterexposure of the remaining photopolymerizable parts in the image areas takes place.
German Utility Models 7,202,150 and 7,805,619 have disclosed that essentially three types of burning-in cabinets are used in practice for printing plates, namely horizontally or vertically operating circulating-air burning-in cabinets or vertically operating burning-in stoves with heating devices based on electromagnetic radiation; heating of the light-sensitive layer by placing the printing plates upon a heated cylinder is also possible.
The burning-in processes disclosed by the state of the art have, however, certain disadvantages. Devices which operate with a naked flame, or in which heated rollers are used, produce only insufficiently reproducible heat distributions with respect to the temperature generated, damage to parts of the printing plates can occur, and no uniform burning-in of the layer is accomplished. Devices in which, for example, heated circulating air is used in a heating cabinet, admittedly lead to a more reproducible temperature adjustment and to an evening-out of burning-in, but long heating-up times before and during the burning-in must be accepted and, especially in the case of printing plate support materials based on organic polymers or composed of aluminum, the mechanical strength (for example tensile strength) of the support materials can be adversely affected. Devices in which electromagnetic radiation with an exclusive or very high IR fraction is used admittedly can lead to a reduction of the treating time from at least 5 to 6 minutes in so-called burning-in cabinets to, for example, about 1 minute, especially if the distance between the light-sensitive layer and the heat source is reduced, but they cause then the undesired side effect of a very severe adverse affect on the mechanical strength of the printing plate support materials.
Although it was already known from the processing of exposed and developed, negative-working printing plates with photopolymerizable compounds in the light-sensitive layer to irradiate these with lamps emitting UV light and IR light in an "afterexposure stage", such a measure pursues different purposes. In fact, these lamps are intended to serve, on the one hand, for drying the material and, on the other hand, for an afterexposure of photopolymerizable fractions still remaining in the image areas. Since, in the case of negative-working light-sensitive layers, the parts of the layer struck by light become less soluble, the irradiation by the lamps having a UV fraction is intended to make those image areas or parts of image areas completely insoluble which are located, for example, in areas adjoining the surface of the support material which are further away from the light source in the actual exposure stage, to the extent that these were not yet or only insufficiently insoluble. This means, however, that this would not be a non-inventive transfer of this teaching to the burning-in of positive-working light-sensitive layers, since precisely opposite prerequisites prevail with these. Initially, it had to be assumed that an irradiation with UV light would have to lead to a higher solubility of the layer areas struck by light.
The invention starts from a device for burning in exposed and developed, positive-working light-sensitive layers containing diazo compounds in the production of printing forms by heating the printing plate by means of electromagnetic radiation having an infrared fraction, a fraction in the visible region and an ultraviolet fraction.